The present invention relates to a ball screw which causes the conversion of rotary motion into linear motion and vice versa and, in particular, to an improvement in a ball screw which is used in a high load applying use.
Conventionally, in a ball screw including a plurality of circulating passages, for example, in a ball screw of a return tube circulation system, in order to reduce the number of steps of working the ball screw, generally, the circulating passages are all arranged in such a manner that the circumferential-direction phases of all circulating passages are identical with each other.
Also, in a ball screw of a circulation frame system, assuming that the number of circulating passages included therein is expressed as n, generally, a plurality of circulating passages are arranged along the axial direction of the ball screw while they are sequentially shifted in phase from each other in the same rotational direction with an equal phase interval equivalent to (1/n) rotation or (2/n) rotation in the circumferential direction.
And, in selecting a ball screw, in order to secure a load capacity corresponding to using conditions such as a load, there are taken various measures: For example, there is selected a ball screw which has a shaft diameter capable of sufficiently withstanding the using conditions, or the number of passages in a ball nut is increased to thereby increase the number of balls, or the lead of the groove of the ball screw is increased to thereby increase the ball diameter.
On the other hand, there is proposed an invention disclosed in Japanese Patent Unexamined Publication No. Hei.10-153245, in which, by equalizing the load distribution of a ball screw, the stress concentration which tends to occur in a ball at a specific position can be relieved, thereby being able to attain the object of providing a ball screw which is compact and has the high load capability without increasing the size of the ball screw.
This conventional invention aims at the following fact. That is, in a ball screw of a return tube circulation system which includes a plurality of circulating passages and also in which the circulating passages are all set identical with each other in the circumferential-direction phases thereof, when an axial-direction load is applied to the ball screw, the arrangement of the effective balls receiving such load is not equalized in the circumferential direction of the circulating passages and the imbalance thereof is relatively large; and, therefore, a high load is applied to part of the balls, that is, to the effective ball portion having a phase in which the number of effective balls is small. According to the cited invention, in a ball screw of a return tube circulation system including three or more circulating passages, the phase of at least one circulating passage is reversed by 180 degrees in the circumferential direction thereof with respect to the remaining circulating passages to thereby equalize variations in the load distribution in the circumferential direction of the circulating passages.
As described above, in the conventional ball screw of a return tube circulation system in which the circumferential-direction phases of all of the circulating passages are arranged identical with each other, in case where an axial-direction load is applied to the ball screw, the arrangement of the effective balls receiving such load is not equalized in the circumferential direction of the circulating passages and also the imbalance thereof is relatively large, so that a high load is applied to the effective ball portion having a phase in which the number of effective balls is small.
On the other hand, as disclosed in the above-mentioned Japanese Patent Unexamined Publication No. Hei.10-153245, in case where the circumferential-direction phase of part of the circulating massages is reversed 180 degrees with respect to the remaining circulating passages, when only the circumferential direction is taken into account, the arrangement of the effective balls receiving the load can be made to approach an equalized state and thus the imbalance hereof becomes small. That is, in this sense, this provides an effective measure to reduce the variations in the load distribution.
However, depending on the manner of arrangement of the circulating passages, the portions respectively having a small number of balls (that is, portions respectively receiving a large load) can be arranged at the mutually opposite positions with respect to the axis of the ball screw; due to such opposite arrangement, there can be generated a moment around a surface perpendicular to the axis of the ball screw; and, such moment around the surface can increase variations in the load distribution along the axial direction of the ball screw. For example, in case where the two mutually adjoining circulating passages share only one portion in which their respective phases are reversed by 180 degrees with respect to each other, with the reversed portion as the boundary, the portions respectively having a small number of balls (that is, portions respectively receiving a large load) are present at the mutually opposite positions with respect to the axis of the ball screw and, due to the presence of a moment around a surface perpendicular to the axis of the ball screw which is generated by such opposite positions, variations in the load distribution along the axial direction of the ball screw are caused to increase.
On the other hand, in the ball screw of a circulation frame system, there are formed three or more circulating passages and, assuming that the number of the circulating passages is expressed as n, the circulating passages are arranged so as to be shifted from one another with an equal phase interval equivalent to (1/n) rotation or (2/n) rotation and return passages are arranged in a spiral manner. That is, since the return passages are arranged symmetrical with respect to the axis of the ball screw when viewed from the axial direction thereof, the ball screw of this system has the advantage of reducing the variations in the load distribution along the circumferential direction of the circulating passages.
However, in the ball screw of this system, because, in the arrangement of the circulating passages along the axial direction of the ball screw, the circulating passages are respectively shifted from one another in the same rotation direction by an equal phase interval in the circumferential direction of the circulating passages, that is, because all of the circulating passages are arranged symmetrical with respect to the axis of the ball screw, there can be generated a moment around a surface perpendicular to the axis of the ball screw, similarly to the previously described ball screw where the two mutually adjoining circulating passages share only one portion in which their respective phases are reversed by 180 degrees with respect to each other. In other words, in the present ball screw, since all of the circulating passages are arranged with an equal phase interval in the circumferential direction of the circulating passages, there is generated a badly-balanced moment around a surface perpendicular to the axis of the ball screw.
The present invention aims at eliminating the drawbacks found in the conventional ball screws. Accordingly, it is an object of the invention to provide a ball screw which can reduce variations in the load distribution caused by the moment around a surface perpendicular to the axis of the ball screw that is generated by shifting the circumferential-direction phases of circulating passages from one another.
In attaining the above object, according to a first aspect of the invention, there is provided a ball screw comprising: a screw shaft including a ball screw groove on the outer surface thereof; at least one ball nut including on the inner surface thereof a ball screw groove opposed to the ball screw groove of the screw shaft; a spiral-shaped passage formed by the ball screw groove of the ball nut and the ball screw groove of the screw shaft; a large number of balls capable of circulating through the spiral-shaped passage; and, a return passage formed on the ball nut for allowing the large number of balls to circulate through the spiral-shaped passage, wherein three or more circulation passages each comprising the spiral-shaped passage and return passage are arranged along the axial direction of the ball screw, wherein, in the circumferential-direction phases of the three or more circulation passages, there are formed at least two portions in which the phases of two mutually adjoining circulation passages are reversed 180 degrees with respect to each other.
According to the first aspect of the invention, by reversing part of the circulation passages 180 degrees with respect to the remaining circulation passages, variations in the load distribution in the circumferential direction of the circulation passages can be improved; and, by setting two portions in which the phases of two mutually adjoining circulation passages are reversed 180 degrees with respect to each other, variations in the load distribution in the axial direction of the ball screw caused by moments around a surface perpendicular to the axis of the ball screw that are generated by shifting the circumferential-direction phases of the circulation passages from each other can be improved.
Now, description will be given below of the reason why the balance of the load distribution can by improved by setting two portions in which the phases of two mutually adjoining circulation passages are reversed 180 degrees with respect to each other.
In the case of a ball screw including only one portion in which the phases of the mutually adjoining circulation passages are reversed 180 with respect to each other, a portion in which the number of balls existing along the circumferential direction of the circulation passages is small (that is, a portion receiving a large load) exists at a position diagonal to the axis of the ball screw, which provides a moment around a surface perpendicular to the axis of the ball screw. The number of moments generated around the surface corresponds to the number of times of 180-degree reversions of the phases between the circulation passages. However, in the case of the mutually adjoining moments, since the directions of the above-mentioned diagonal angles are opposite, the moments provide mutually opposite moments and thus they can cancel or weaken each other. In this manner, by providing two or more phase reversion portions, the balance between the above moments caused by setting the circumferential-direction phase difference between the mutually adjoining circulation passages can be improved, which can reduce the variations in the axial-direction load distribution caused by such moments.
In this case, preferably, in order that the mutually canceling moments can paired with each other, the mutually opposite-direction moments may be generated so as to be equal in number, and the number of portions in which the 180 deg. reversion is made may be set in an even number (especially, two).
Next, according to the second aspect of the invention, there is provided a ball screw comprising: a screw shaft including a ball screw groove on the outer surface thereof; at least one ball nut including on the inner surface thereof a ball screw groove opposed to the ball screw groove of the screw shaft; a spiral-shaped passage formed by the ball screw groove of the ball nut and the ball screw groove of the screw shaft; a large number of balls capable of circulating through the spiral-shaped passage; and, a return passage formed on the ball nut for allowing the large number of balls to circulate through the spiral-shaped passage, wherein three or more circulation passages each comprising the spiral-shaped passage and return passage are arranged along the axial direction of the ball screw, and a phase difference is set between part or all of mutually adjoining circulation passages in the circumferential direction thereof, wherein the three or more circulation passages are arranged so as to have an almost surface symmetry with respect to a surface passing through the axial-direction center of the whole of the three or more circulation passages or passing through the neighboring portion of such axial-direction center and being perpendicular to the axis of the ball screw.
By the way, in order to control the variations in the circumferential-direction load distribution, preferably, the above-mentioned circumferential-direction phase difference may be set for 180 degrees or may be set for an equal phase interval such as (m/the number of circulation passages, where m=integral number) so that the positions of the return passages of all of the circulation passages have a point symmetry with respect to the axis of the ball screw when viewed from the axial direction of the ball screw.
According to the second aspect of the invention, since a circumferential-direction phase difference is set between part or all of the circulation passages, even when there exist moments around a surface perpendicular to the axis of the ball screw, the above phase differences are set so as to have a surface symmetry with the axial-direction center of the whole of the circulation passages as the boundary thereof, two moments opposite in direction and equal in intensity are present while they are paired with each other. As a result of this, the paired moments cancel each other so that the moments around a surface perpendicular to the axis of the ball screw generated due to the arrangement of the three or more circulation passages are allowed to balance well, thereby being able to reduce the variations in the axial-direction load distribution caused by the moments.
For example, in case where the second aspect of the invention is applied to a ball screw in which a phase difference of 180 degree is set in part of the mutually adjoining circulation passages, there exist an even number of 180-deg. phase reversion portions and they are arranged symmetrical with respect to the axial-direction center of the whole of the three or more circulation passages.
Next, according to the third aspect of the invention, there is provided a ball screw comprising: a screw shaft including a ball screw groove on the outer surface thereof; at least one ball nut including on the inner surface thereof a ball screw groove opposed to the ball screw groove of the screw shaft; a spiral-shaped passage formed by the ball screw groove of the ball nut and the ball screw groove of the screw shaft; a large number of balls capable of circulating through the spiral-shaped passage; and, a return passage formed on the ball nut for allowing the large number of balls to circulate through the spiral-shaped passage, wherein three or more circulation passages each comprising the spiral-shaped passage and return passage are arranged along the axial direction of the ball screw, and a phase difference is set between part or all of mutually adjoining circulation passages in the circumferential direction thereof, wherein, in at least one portion between the mutually adjoining circulation passages, the circumferential-direction phase interval thereof is offset from the equal phase interval.
According to the third aspect of the invention, as in a ball screw of a circulation frame system, in a ball screw in which, assuming that a circulation route comprises three or more circulation passages and the number of circulation passages is expressed as n, the circulation passages are arranged at a phase difference of an equal phase interval such as (1/n) rotation or (2/n) rotation to thereby control variations in the circumferential-direction load distribution, by changing the phase interval of at least one portion thereof (that is, by offsetting the phase interval from the equal phase interval), the balance between the moments around a surface perpendicular to the axis of the ball screw is improved to thereby be able to reduce the variations in the axial-direction load distribution caused by the present moments.
To determine the above offsetting quantity, an analysis for improving the direction balance of the moments may be made and the offsetting quantity may be determined in accordance with the results of the analysis.
By the way, even in case where the phase interval of part of the circulation passages is changed, preferably, the phases of the remaining circulation passages may be set in such a manner that the return passages of all of the circulation passages are arranged so as to have a point symmetry with respect to the axis of the ball screw when viewed from the axial direction of the ball axis.